The present invention relates generally to an actuator for a control system in a drive line apparatus, and more particularly to a clutching actuator for selectively controlling a component in a transfer case.
Many vehicles commonly drive either the front wheels or the rear wheels, which are powered by drive torque produced by the engine and distributed through the transmission. The drive train in many light duty trucks and sport utility vehicles includes a transfer case for transmitting drive torque from the engine and transmission to all four wheels, thereby establishing a four-wheel drive mode of operation. Drivers of these types of vehicles are provided with the option of selecting between two-wheel drive and four-wheel drive modes of operation. To accommodate differing road surfaces and conditions, many transfer cases are equipped with a gear reduction unit that can be selectively shifted by the vehicle's driver to further establish a four-wheel high-range and four-wheel low-range drive modes.
Early four-wheel drive vehicles required that the driver stop the vehicle, leave the vehicle and manually lock wheel hubs to engage the four-wheel drive mode. Current technology has evolved to allow the driver to engage four-wheel drive mode without leaving the cab of the vehicle. This can be accomplished through mechanical methods (i.e., a gearshift) or electronic methods (i.e., motor control). Furthermore, advanced controls allow electronic "on-the-fly" or "on-the-move" engagement of the four-wheel drive including selection between four-wheel high-range and low-range driving modes. Gear reduction units equipped with synchronizer clutches permit the vehicle's operator to shift from four-wheel low-range drive mode into four-wheel high-range drive mode without stopping the vehicle and alleviate the inconveniences of early systems. For example, U.S. Pat. No. 5,054,335 discloses a transfer case equipped with a synchronized range shift arrangement for "on-the-fly" shifting of a layshaft-type gear reduction unit. Alternatively, U.S. Pat. No. 5,346,442 discloses a transfer case having a synchronized range shift arrangement for "on-the-fly" shifting of a planetary-type gear reduction unit. In addition, U.S. Pat. No.5,655,986 discloses a transfer case equipped with a planetary-type gear reduction unit that permits synchronized shifting between high-range drive mode and low-range drive mode. Pending U.S. patent application Ser. No. 09/237,179 discloses a transfer case shift control system using an automatic shutdown relay circuit for on the move shifting between high-range and low-range drive modes. U.S. Pat. No. 5,346,442 and pending application Ser. No. 09/237,179 are commonly assigned to the assignee of the present invention and their disclosures are hereby expressly incorporated by reference herein.
In addition to the above-noted mechanically synchronized range shift system, it is also known to electronically control the drive train to provide "on-the-fly" range .shifting of transfer cases. For example, an electronically shifted two-speed transfer case is disclosed in U.S. Pat. No. 5,522,777 as having a transfer case control module which receives input signals from the engine control module relating to the current transmission gear, engine speed and vehicle speed. The transfer case control module uses these input signals to generate control signals that feed back to the engine control module for use in regulating the vehicle's operation and to accommodate "on-the-fly" shifting. Specifically, the engine control module will control the engine's fuel system to modify the engine speed and/or shift the transmission gear to match the rotary speed of the transmission output with that of the transfer case output prior to actuation of the range shift mechanism.
Four-wheel drive vehicles may also employ electronic traction control systems which aid the driver in maintaining vehicle control under a wide range of terrain and driving conditions by selectively controlling the distribution of torque between the front and rear wheels through the use of a clutch assembly. Under moderate, normal driving conditions it is not desirable to use full-time four-wheel drive in a vehicle; however, changing road conditions and weather conditions make selective engagement of four-wheel drive advantageous to maintain proper control of the vehicle and to provide additional traction. Traction control systems, in part, work by sensing the speed differential between both axles. If an excessive difference exists, the torque distribution is modified to provide additional traction and control to a particular axle and thus the front or rear wheels. When the speed differential drops and the extra traction and control is no longer needed, the four-wheel drive automatically disengages and the vehicle returns to two-wheel drive. These systems have gained popularity, in part, due to their versatility and programmability.
Many vehicles now include anti-lock braking systems (ABS) require integration with electronic traction control systems. Anti-lock braking systems sense the speed and traction of each wheel during braking and apply a braking algorithm to maintain control and traction of the vehicle and to prevent skidding. An ABS algorithm may vary the braking force on the wheels individually. It is not desirable to have the front and rear drive lines mechanically connected when braking a single wheel or drive line since the mechanical connection would impede the effectiveness of the braking control by allowing the braking mechanism on one wheel to have a greater effect on the other wheels. To effectively control braking in anti-lock braking systems, traction control systems must be able to quickly disengage from the four-wheel drive mode of operation in order to allow for ABS yaw control and optimum ABS braking performance. Therefore, the response time of the clutch assembly must be sufficient fast to be compatible with ABS algorithms that require quick disengagement.
Current clutch control systems, in addition to having insufficient response times, use motors to engage and disengage the clutch assembly that require high torque and, therefore, also draw high currents. These currents can be in the tens of amps range. Because of the high currents produced by the pulse width modulation (PWM) motors used, the current systems produce substantial electromagnetic interference (EMI). EMI can adversely affect the vehicle's various electrical systems, especially adjacent systems. Many countries strictly regulate EMI, and, therefore, another a strong need exists to reduce EMI emissions from their present levels.
Current actuator system selectively engage the four-wheel drive systems through the bidirectional operation of a drive motor. The four-wheel drive mode is engaged by driving a motor to move a sector shaft in one direction. The four-wheel drive mode is disengaged by driving the motor in the reverse direction and, therefore, reversing the motion of the sector shaft. Current system response times, are limited in part, by the time it takes to reverse the motor. The motors in current actuator systems cannot reverse fast enough to meet the demands of the new traction control systems. In addition to the performance limitations, the reversal of motors causes substantial EMI and current draws. Therefore, a need exists to increase the response times of current actuator systems.
In view of the substantial interest in "on-the-fly" transfer case actuator systems, a recognized need exists to develop an actuator system for four-wheel drive vehicles equipped with ABS systems and electronic traction control systems to further advance the current technology.